Fluorinated polymers have a unique position in the plastics industry due to their chemical inertness, heat resistance, electrical insulation, low coefficient of friction and water repellancy. Notwithstanding the enormous advantages of fluorinated polymers their inert chemical nature, especially of perfluorinated polymers, makes them extremely hard to bond with adhesives. Fluoropolymers are not amenable to the usual surface modification techniques that have enabled adhesive bonding of hydrocarbon polymers. Surface treatments such as corona discharge and flame treatment, which are well known in the art for introducing bondable polar groups to the surface layers of, for example, polyolefins are ineffective with fluorinated polymers.
The presence of fluorine seems to markedly affect the chemical reactions occurring in polymer surface layers upon surface treatments. It is known that oxygen-containing or nitrogen-containing groups can be incorporated into the surface of, for instance, PTFE (Liston et al., Journal of Adhesion Science and Technology, 7, (1993)) and FEP (Xie et al., Journal of Adhesion Science and Technology, 6 , 1411 (1993)) and the modified surfaces can be wetted, but adhesion is nevertheless poor. This poor adhesion has been attributed to a weak sub-surface structure of PTFE (Liston et al., Journal of Adhesion Science and Technology, 7, (1993)). Moreover the effects of ammonia plasma treatment are greatly reduced over a period of a few days due to surface layer motability (Xie et al).
The surface layers of perfluorinated polymers can be modified also by exposure to an etching solution that contains sodium naphthalene complex. PTFE surfaces etched thus can be adhesively bonded. The "sodium etch" method suffers, however, from considerable disadvantages. The high reactivity of the etch solution makes it dangerous to handle, requiring cumbersome precautions, and causes substantial disposal problems.
Attempts have been made to modify perfluorinated polymers by surface treatment methods such as corona discharge, flame treatment, and low pressure gas plasma (glow discharge) treatments, which pose far lower environmental hazards and are well established. The former two treatments are oxidative in nature and have not given satisfactory bond strengths. Low pressure plasma methods are more versatile in the choice of gas and enable introduction of various chemical groups onto polymer surfaces. However, in spite of many studies on, for instance, the introduction onto PTFE surfaces of amine groups to enable covalent adhesive bonding with cyanoacrylate or epoxy adhesives, no satisfactory process has been found; the bond strength was always found to be considerably below the cohesive strength of the material.
To improve subsequent adhesion perfluorinated polymers have been glow discharge treated in hydrogen/nitrogen mixtures (JP-A-6220231) (etching), in silane (JP-A-3164246), fluorocarbons or chlorfluorocarbons (DE-A-3408837) and in Ar, N.sub.2, O.sub.2, He or air (JP-A-3064382). Surface modification of PTFE by ammonia plasma treatment and its effect on bonding to nitrile rubber using a phenol-type adhesive was reported by Inagaki et al in J Adhesion Sci Technol Vol 3, no. 8, pp 637-649 (1989). PTFE strips were given a ten minute treatment in a continuous 20 kHz 150 mA plasma in 26Pa NH.sub.3 atmosphere at various sample temperatures; only when the sample temperature reached over 200.degree. C. were bonds of strength exceeding the cohesive strength of the PTFE formed.